You can improve your windsurfing technique by truly understanding Apparent Wind.
Apparent Wind is a term that most sailors have heard about but very few really
understand. Yet, the behaviour of the apparent wind under different
conditions has a profound effect on the performance of all sailing craft .
This is especially true in the case of windsurfing where the speed of the
board very often exceeds that of the true wind. A sound understanding
of the principle of apparent wind will help in improving a sailor's technique
on all points of sailing, whether it is in sailing upwind, reaching or
broadreaching. An awareness of the effects of apparent wind is also
important when performing actions like pumping and gybing.

If you are riding a bicycle on a still day, you will feel the wind in your face although there is actually no wind blowing. This wind that you feel is the apparent wind that is generated by the movement of the bicycle and the rider through the air. If there is a headwind, then the wind in your face will be stronger and conversely, if there is a tailwind then you will feel less wind in your face. This then is the effect of apparent wind.

In sailing, the sails are always trimmed to suit the wind direction for maximum drive. As the boat picks up speed, the strength and direction of the wind on the sails change because of the effect of the apparent wind. The wind becomes stronger and the sails have to be sheeted closer in to maintain correct trim. The effect becomes more pronounced as the speed of the boat increases in relation to the true wind. Because windsurfers very often sail at speeds considerably faster that the true wind, the trim of a windsurfer's sail bears no relation to the direction of the true wind. When a windsurfer is sailing on a broad reach at speed, his sail is sheeted so close in that he appears to be sailing upwind. This is because the apparent wind has moved well forward and is now coming from the front quarter.

Understanding this behaviour of the wind will help in early planing. Present day boards are designed so that they will start planing when their speed exceeds about 7 knots. The speed then shoots up very quickly in excess of about 13 knots. For such boards, it is not possible to maintain an intermediate speed between these two values. An experienced windsurfer will recognize when he is on the verge of planing. He will then only have to give a couple of short pumps on the sail to give the board a nudge to push it past the threshold planing speed. As the board speeds up, the apparent wind will move forward and increase in strength. To take advantage of this, the sail has to be sheeted in and held there. If the wind should weaken, the board will slow down and come off the plane. The sail has then to be sheeted out to maintain correct trim as the apparent wind will have moved back again.

I have written a more in-depth explanation of how the apparent wind changes on different points of sailing. It is rather technical and requires some understanding of vector diagrams. It can be found here: True Wind & Apparent Wind